Enols (also known as
alkenols) are
alkenes with a
hydroxyl group affixed to one of the carbon atoms composing the
double bond. Enols and
carbonyl compounds (such as
ketones and
aldehydes) are in fact
isomers; this is called
keto-enol tautomerismThe enol form is shown on the left. It is usually unstable, does not survive long, and changes into the keto (ketone) form shown on the right. This is because oxygen is more electronegative than carbon and thus forms stronger multiple bonds. Hence, a carbon-oxygen (carbonyl) double bond is more than twice as strong as a carbon-oxygen single bond, but a carbon-carbon double bond is weaker than two carbon-carbon single bonds.
Only in 1,3-dicarbonyl and 1,3,5-tricarbonyl compounds does the (mono)enol form predominate. This is because the resonance and intermolecular hydrogen bonding that occurs in the enol form is not possible in the keto form. Thus, at equilibrium, over 99% of propanedial (OHCCH2CHO) molecules exist as the monoenol. The percentage is lower for 1,3-aldehyde ketones and diketones. Enols (and enolates) are important intermediates in many organic reactions.
The words enol and alkenol are portmanteaux of the words alkene (or just -ene, the suffix given to alkenes) and alcohol (which represents the enol's hydroxyl group).
